The present invention pertains to cardiac pacing systems for providing multi-site pacing in a single heart chamber or multi-chamber pacing including AV sequential pacing and sensing in at least one upper and one lower heart chamber and/or bi-atrial pacing or bi-ventricular pacing involving pacing and sensing in two, three, or four heart chambers and particularly to employing miniaturized electrical isolation circuitry at the inputs of least one of the sense amplifiers associated with a site or heart chamber to improve the sensing of cardiac depolarizations following delivery of a pacing pulse at another site or heart chamber.
The cardiovascular system provides oxygenated blood to various structures of the body. In a normally functioning heart, the body""s demand for oxygenated blood varies, and the heart responds by increasing or decreasing its rate and force of contraction to meet the demand. An electrical signal generated by the sinus node in the upper right atrial wall near the base of the heart is conducted through the upper heart chambers, i.e., the right and left atria, and causes them to contract in a synchronous manner. The contraction of the upper heart chambers forces blood pooled therein through open heart valves and into the right and left ventricles or lower heart chambers. The atrial electrical depolarization wave arrives at the AV node superior to the ventricles and triggers the conduction of a ventricular depolarization wave down the bundle of His in the septum between the right and left ventricles to the apex of the heart. The ventricles contract after a brief atrio-ventricular (AV) delay time following the sinus node depolarization as the depolarization wave then advances superiorly, posteriorly, and anteriorly throughout the outer ventricular wall of the heart. The lower heart chambers contract and force the blood through the vascular system of the body. The contraction of the right and left ventricles proceeds in an organized fashion which optimizes emptying of the ventricular chambers. The synchronous electrical depolarization of the atrial and ventricular chambers can be electrically sensed and displayed, and the electrical waveform is characterized by accepted convention as the xe2x80x9cPQRSTxe2x80x9d complex. The PQRST complex includes the P-wave, corresponding to the atrial depolarization wave, the R-wave, corresponding to the ventricular depolarization wave, and the T-wave which represents the re-polarization of the cardiac cells.
Various disease mechanisms cause conduction disturbances which interfere with the natural conduction system of the heart and affect the heart""s ability to provide adequate cardiac output to the body. In certain disease mechanisms, the sinus node fails to depolarize and commence the P-wave as rapidly as required to satisfy the demand for oxygenated blood, or the atria may spontaneously depolarize at rates that are well in excess of the ability of the ventricles to respond. In these situations, the ventricles may compensate by depolarizing spontaneously from ectopic depolarization sites. In other cases where the SA node operates correctly, 1:1 atrial and ventricular depolarization synchrony is lost because the AV node fails to respond to all P-waves or a defect in the bundle of His interferes with the conduction of the ventricular depolarization. In all of these cases, the ventricles may contract at an inadequate rate to provide adequate cardiac output.
When the atria or ventricles contract too slowly, the patient may be a candidate for implantation with a cardiac pacemaker for restoring the heart rate by applying pacing pulses to the heart chamber that is malfunctioning at a pacing rate that restores adequate cardiac output. Modern implantable cardiac pacemakers comprise an implantable pulse generator (IPG) and a lead or leads extending from the IPG to pace/sense electrode or electrodes located with respect to the heart chamber to deliver the pacing pulses and sense the P-wave or R-wave. Typically, the leads are transvenously introduced into the particular heart chamber via the superior vena cava and right atrium, and the pace/sense electrodes are maintained in contact with the pace/sense electrode or electrodes located with respect to the heart chamber to deliver the pacing pulses and sense the P-wave or R-wave. Typically, the leads are transvenously introduced into the particular heart chamber via the superior vena cava and right atrium, and the pace/sense electrodes are maintained in contact with the heart tissue by a fixation mechanism at the distal end of the lead. However, leads may be placed subcutaneously between the IPG and the exterior of the heart, and the pace/sense electrodes attached to the epicardium at the desired sites. Moreover, endocardial coronary sinus leads are introduced through the right atrium into the coronary sinus and the great vein to locate pace/sense electrodes in proximity to the left atrium or the left ventricle.
A single chamber, demand pacemaker is implanted to supply pacing pulses to a single upper or lower heart chamber, typically the right atrium or right ventricle, in response to bradycardia of the same chamber. In an atrial, demand pacemaker operating in the AAI pacing mode, an atrial pacing pulse is delivered to the atrial pace/sense electrodes by the IPG if a P-wave is not sensed by an atrial sense amplifier coupled to the atrial pace/sense electrodes within an atrial escape interval (Axe2x80x94A interval) timed by an atrial escape interval timer. In a ventricular, demand pacemaker operating in the VVI pacing mode, a ventricular pacing pulse to the ventricular pace/sense electrodes if an R-wave is not sensed by a ventricular sense amplifier coupled to the ventricular pace/sense electrodes within a ventricular escape interval (Vxe2x80x94V interval) timed by a ventricular escape interval timer.
A dual chamber, demand pacemaker is implanted to supply pacing pulses when required to one upper heart chamber and to one lower heart chamber, typically the right atrium and right ventricle. In a dual chamber, demand pacemaker operating in the DDD pacing mode, both the AAI and VVI pacing modes are followed under the above defined conditions. A ventricular pacing pulse is delivered to the ventricular pace/sense electrodes if an R-wave is not sensed by the ventricular sense amplifier coupled thereto within an AV time interval timed from the sensing of a P-wave by the atrial sense amplifier.
Over the years, it has been proposed that various conduction disturbances involving both bradycardia and tachycardia of a heart chamber could benefit from stimulation applied at multiple electrode sites positioned in or about it in synchrony with a depolarization which has been sensed at least one of the electrode sites. In addition, it has been proposed to employ pacing to compensate for conduction defects and in congestive heart failure where depolarizations that naturally occur in one upper or lower chamber are not conducted quickly enough to the other upper or lower heart chamber. In such cases, the right and left heart chambers do not contract in optimum synchrony with each other, and cardiac output suffers due to the timing imbalance. In other cases, spontaneous depolarizations of the left atrium or left ventricle occur at ectopic foci in these left heart chambers, and the natural activation sequence is grossly disturbed. In such cases, cardiac output deteriorates because the contractions of the right and left heart chambers are not synchronized sufficiently to eject blood therefrom.
In patients suffering from congestive heart failure, the hearts become dilated, and the conduction and depolarization sequences of the heart chambers may exhibit Intra-Atrial Conduction Defects (IACD), Left Bundle Branch Block (LBBB), Right Bundle Branch Block (RBBB), and Intra Ventricular Conduction Defects (IVCD). Single and dual chamber pacing of the right atrium and/or right ventricle can be counterproductive in such cases, depending on the defective conduction pathway and the locations of the pace/sense electrodes.
A number of proposals have been advanced for providing pacing therapies to alleviate these conditions and restore synchronous depolarization of right and left, upper and lower, heart chambers as described in commonly assigned U.S. Pat. No. 5,902,324 and references disclosed therein. Typically, the right atrium is paced at expiration of an Axe2x80x94A escape interval, and the left atrium is simultaneously paced or synchronously paced after a short delay time. Similarly, the right ventricle is paced at expiration of a Vxe2x80x94V escape interval, and the left ventricle is simultaneously paced or synchronously paced after a short delay time. Some of these patents propose limited forms of DDD pacing having xe2x80x9cbi-ventricularxe2x80x9d or xe2x80x9cbi-atrialxe2x80x9d demand or triggered pacing functions. A pacing pulse delivered at the end of an escape interval or at the end of an AV delay (a xe2x80x9cpaced eventxe2x80x9d) triggers the simultaneous or slightly delayed delivery of the pacing pulse to the other heart chamber.
The above-referenced ""324 patent proposes pacing a right heart chamber (RHC) or left heart chamber (LHC) at the end of the escape interval or an AV delay. Pacing in the other of the RHC or LHC is inhibited if a conducted depolarization is detected in that other heart chamber within a physiologic time related to the location of the pace/sense electrodes and referred to therein as a conduction delay window (CDW).
These approaches show promise in restoring the synchronous contractions of the right and left heart chambers in diseased hearts having significant conduction disturbances of the right and left heart depolarization waves but fail to preserve right and left heart synchrony in a physiologic manner. Significant conduction disturbances between the right and left atria can result in left atrial flutter or fibrillation that can be suppressed by pacing the left atrium synchronously with right atrial pacing or sensing of P-waves. And, particularly in patients suffering from heart failure, left atrial and left ventricular cardiac output can be significantly improved when left and right chamber synchrony is restored.
All of the above-described pacing systems operate in demand and/or triggered and/or synchronous modes that depend upon the ability to accurately sense P-waves and/or R-waves at one or more site or heart chamber in the presence of electromagnetic interference (EMI) and in as short a time as possible following delivery of a pacing pulse. A xe2x80x9cpacing channelxe2x80x9d is defined for each pacing site of a single chamber, multi-chamber or multi-site pacing system, by the lead, the pacing output circuit, the sense amplifier, and associated circuitry coupled to the lead extending to the pace/sense electrode pair for that site. The inputs of the sense amplifier and an output capacitor of the output circuit are commonly coupled to the respective pace/sense electrode pair. Pacing pulses are delivered to the pair of pace/sense electrodes of the pacing channel wherein at least one of the pace/sense electrodes is at pacing site and the other, indifferent, pace/sense electrode is either located on the lead close thereto to provide bipolar pacing and sensing or located at a more remote location, e.g., the case or can of the IPG, to provide unipolar pacing and sensing. In either case, the indifferent pace/sense electrodes of all of the pacing channels are all typically electrically connected in common and with a common ground circuit of the pacing circuitry. The battery is also typically connected to the common ground circuit. Low resistance coupling components of the pacing output circuits can also conduct leakage currents to the active pace/sense electrodes of the pace/sense electrode pairs of two or more pacing channels.
For a number of reasons, it is often difficult to sense P-waves, R-waves or other signals of the PQRST complex caused by a paced depolarization or a spontaneous depolarization for a time following delivery of a pacing pulse in the same channel or in another channel. The lead conductors, the xe2x80x9celectrode-tissue interfacexe2x80x9d of the pace/sense electrode pair with cardiac tissue or fluid, and the mass of cardiac tissue or fluid between the pace/sense electrode pair comprise a capacitive-resistive reactance presented to the output of the output circuit and the input of the sense amplifier of the pacing channel. Pacing pulses are typically delivered by partial discharge of a charged output capacitor into the capacitive-resistive reactance of the pacing channel coupled directly therewith, and the output capacitor recharges during the interval between pacing pulses. The pacing pulse energy is directly delivered to the xe2x80x9csame channelxe2x80x9d, pace/sense electrode pair as intended, but leakage current or xe2x80x9ccross-talkxe2x80x9d can be conducted xe2x80x9ccross-channelxe2x80x9d through the pacing system common ground and coupling components to the pace-sense electrode pairs or the non-paced pacing channels.
The discharge of an output capacitor results in same-channel or cross-channel after-effects due to the disruption of the electrical equilibrium condition at the tissue-electrode interface by the discharge current or leakage current, respectively, resulting in polarization of the tissue""s intrinsic dipole moments. These stimulation caused xe2x80x9cafter-potentialsxe2x80x9d manifest themselves to traditional pacemaker sense amplifiers coupled to a pace/sense electrode pair as decaying voltage signals that persist for a period of time following delivery of pacing pulses until the electrical equilibrium condition is restored. These after-effects interfere with the sense amplifier""s ability to sense depolarizations of the heart closely following or caused by delivery of stimulation pulses.
Various attempts have been made in the prior art to counteract the after-potentials of the pacing pulse and simultaneously recharge the output capacitor by means of a xe2x80x9cfast rechargexe2x80x9d current delivered through the pace/sense electrode pair following the trailing edge of the pacing pulse, as exemplified by U.S. Pat. Nos. 4,476,868, 4,406,286, 3,835,865 and 4,170,999. However, simply passing sufficient current through the electrode-tissue interface to recharge the output capacitor does not necessarily return the electrode-tissue system to its prior electrical equilibrium condition. Alternatively, it has been suggested to counteract the after-effects of delivery of a stimulation pulse by simply tying the electrodes involved in delivery of the pulse together following delivery of the pulse, as disclosed in U.S. Pat. No. 4,498,478 or by means of a train of low energy pulses as disclosed in U.S. Pat. No. 4,811,738.
As set forth in the ""324 patent and in commonly assigned U.S. Pat. No. 5,156,149, very high impedance P-wave and R-wave sense amplifiers that do not substantially load the signal source have been employed in pacing systems since the time that integrated circuit (IC) technology was adopted. The sense amplifier has undergone steady development and refinement as reflected by the teachings of commonly assigned U.S. Pat. Nos. 4,275,737, 4,379,459, and 4,649,931. However, the underlying design philosophy, requiring high impedance and high gain in order to sense the low level signal generated by the heart, has remained the same over the years. Band pass filters, time domain filtering, and amplitude threshold comparison continue to be employed to discriminate a P-wave or R-wave from EMI and same channel and cross-channel after-potentials persisting from a prior pacing pulse applied to the same channel or cross-channel pace/sense electrode pair. The prior art, high input impedance, sense amplifier circuits are easily saturated by the pacing pulse delivered between the pace/sense electrodes coupled to the input terminals of the sense amplifier or delivered between other chamber or other site pace/sense electrodes.
When the first AV-sequential, DVI dual chamber pacing systems were developed as shown in U.S. Pat. Nos. 3,757,791, 3,766,413, and 3,814,106, it was found convenient to electrically isolate the atrial sense electrodes from the ventricular pace/sense electrodes through an isolation transformer. However, this approach employing relatively bulky wire wound transformers was abandoned with adoption of IC fabrication technology enabling the miniaturization of the IPG circuitry and the inability of obtaining sufficiently small and reliable discrete component transformers.
It has also been suggested to minimize interaction between the sensing and pacing functions by dedicating separate lead conductors and electrodes to the pacing pulse output circuit and the sense amplifier input terminals as described for example in commonly assigned U.S. Pat. No. 4,310,000. However, lead size and limited IPG can feedthrough space and connector size considerations have to this time dictated use of IPG connector and lead systems having pace/sense electrodes that are shared as described above.
Presently, the sense amplifier input terminals are typically un-coupled from the pace/sense electrodes for a predetermined xe2x80x9cblankingxe2x80x9d period started on delivery of a pacing pulse across the same channel pace/sense electrode pair or on delivery of a pacing pulse to the pace/sense electrode pair of any other pacing channel to help prevent saturation due to the pacing pulse energy. The blanking period typically extends for a further time period to allow the after-potentials at the electrode-tissue interface to dissipate sufficiently to reliably sense the cardiac signal of interest. The blanking switches typically comprise a single FET switch that is connected in series with one or both of the sense amplifier inputs that are normally closed but are opened during the blanking period and/or another FET switch that is coupled across the input terminals that is normally open but is closed during the blanking period. Exemplary blanking circuitry is disclosed in commonly assigned U.S. Pat. No. 4,401,119, for example. The typical same channel blanking period is about 100 msec in duration and the typical cross-channel blanking period is about 30 msec in contemporary pacemaker IPGs.
Before the adoption of IC fabrication, sense amplifiers were formed of rather bulky discrete components assembled as hybrid circuitry. Blanking was effected in single chamber pacing systems by preventing the sense amplifier output signal from being used by downstream pacemaker circuitry. Sense amplifiers and pacing output circuitry have been fabricated employing discrete components and bipolar ICs mounted in a hybrid package. Timing and control functions have been implemented employing digital IC fabrication techniques, in recent years incorporating a microprocessor, memory and associated components forming a micro-computer and mounted on a substrate. Most recently, linear and sub-micron CMOS fabrication techniques have been adopted that consolidate all of the pacing IPG circuitry except for certain discrete components on a single chip. This has made it more difficult to shorten blanking periods because of reduced voltage breakdown and circuit cross-talk.
In the context of bi-atrial or bi-ventricular sensing and pacing systems described above in reference to the ""324 patent, it would be desirable to program the CDW for sensing a conducted depolarization in one heart chamber responding to a pace pulse or sensed event in the other chamber between 5-10 msec and 100 msec, for example. The CDW time depends on the physical locations of the right and left chamber pace/sense electrodes and normal conduction time delays therebetween. In this range, the after-potentials from a pace pulse delivered in the other chamber and reflected to the pace/sense electrodes in the chamber being timed will obscure any underlying evidence of a conducted cardiac depolarization occurring within the CDW time. Use of the typical 100 msec blanking period to overcome the after-potentials problem would prevent the sense amplifier from sensing the conducted depolarization wave.
In the ""324 patent, it is suggested that a xe2x80x9cfield density clampxe2x80x9d circuit be employed that treats the pace/sense electrode pair as two electrode poles and loads the two electrode poles to measure the amount of current injected into the lead system by a passing wavefront. It is asserted that the field density clamp detection system is especially suited to systems in which pacing and sensing functions share electrode poles since this detection strategy is relatively insensitive to the so called xe2x80x9celectrode polarizationxe2x80x9d effects caused by the delivery of pacing energy to excitable tissue, through a lead system. In operation, the active circuitry establishes and maintains the electric field density required to maintain an equilibrium condition between the two poles. The field perturbation caused by the passing wavefront is nulled out by the active circuitry which attempts to balance the potentials at the electrodes. The amount of current supplied to the electrode surfaces through a virtual load, that is required to maintain this null condition, is monitored and forms the basis for the detection of the passing depolarization wavefront. It is preferred to also monitor the voltage across the virtual load and multiply it with the current measurement to characterize the power delivered to the electrode system by the passing depolarization wavefront. Unfortunately, this sensing concept inherently involves an unacceptable current drain at high sensing rates, e.g., during tachyarrhythmia episodes, as compared to conventional sense amplifiers and is susceptible to EMI from several sources.
In all of the above-described examples and in others that will occur to those of skill in the art, it remains desirable to reduce the blanking periods of sense amplifiers employed to sense a conducted or natural cardiac depolarization across a pair of pace/sense electrodes after delivery of a pacing pulse to the same or a different pair of pace/sense electrodes. The reduction in the blanking periods must be effected in a manner that does not increase the size of the pacing system or increase current consumption from the IPG battery.
The present invention is therefore directed to reducing the blanking periods of sense amplifiers employed to sense a cardiac depolarization across a pair of pace/sense electrodes after delivery of a pacing pulse to the same or a different pair of pace/sense electrodes in at least a two site or chamber pacing system. The present invention incorporates monolithic isolation circuit means comprising an output current loop coupled with a pair of pace/sense electrodes and an input current loop coupled to the sense amplifier inputs, the input and output current loops formed as integrated circuit conductors and functioning as isolated current replicators of sensed cardiac depolarizations. In a multi-site or multi-chamber pacing system having N sense amplifiers in N sense amplifier channels, Nxe2x88x921 isolated current replicators are in circuit between the pace/sense electrodes and the sense amplifier inputs of up to Nxe2x88x921 sense amplifiers.
Preferably, each isolated current replicator is also in circuit between the pacing pulse generator and the pair of pace/sense electrodes of the channel. The output current loop is coupled with the pair of pace/sense electrodes of the channel, and the input current loop is coupled with both the input of the sense amplifier and the output of the pacing pulse generator. In this embodiment, pacing trigger pulses delivered to the input current loop are replicated in the output current loop and delivered to the pace/sense electrodes, whereas cardiac signals traversing the pace/sense electrodes and the output current loop are replicated in the input current loop and provided to the sense amplifier.
The input current loop and the output current loop are isolated from one another so that the output current loop and the components coupled therewith, including the pace/sense electrode pair of the pacing channel are uncoupled from the pacing circuitry and isolated from leakage currents accompanying delivery of a pacing pulse to a pace/sense electrode pair of another pacing channel. The output current loop is isolated from the pacing circuitry coupled to the input current loop to prevent cross-channel leakage current accompanying delivery of a pacing pulse in another pacing channel from being applied to the pace/sense electrode pair coupled with the output current loop. Thus, after-potentials do not develop on the isolated pace/sense electrode pair, and the blanking period can be substantially reduced.
Preferably, a first blanking period is commenced for each sense amplifier coupled with a current replicator input current loop when a pacing pulse is delivered to the same channel pace/sense electrode pair that is connected to the input current loop. A second blanking period is commenced when the cross-channel pacing pulse is applied to a different pace/sense electrode pair than is connected to the input current loop. In the latter case, the blanking period can be set to zero or the width of the pacing pulse and associated recharge time or about 5 msec to about 10 msec. In the former case, the blanking period can be set to a range of about 50 msec to about 100 msec.
In one embodiment, the isolated current replicator is formed employing giant magnetoresistive (GMR) elements, each GMR element comprising a GMR inductor associated with a GMR resistor fabricated in monolithic form isolated planar cells and incorporated into conventional VLSI circuitry. The input and output current loops are formed with GMR inductors associated with GMR resistors that are in turn coupled in a bridge circuit with the inputs of an operational amplifier. The output current loop is coupled to the output of the operational amplifier and with the pair of pace/sense electrodes of the pacing channel. The input current loop is coupled to the inputs of the sense amplifier and the output of a pacing pulse generator of the pacing channel.
In a further embodiment, the isolated current replicator is formed of a micro-mechanical fabricated (MEMS) isolation transformer comprising low-loss input and output coils separated by an insulation layer that isolates the input coil from the output coil.
The present invention can be implemented in various multi-site and multi-chamber pacing systems, preferably in multi-chamber pacing systems providing pacing and sensing in an upper and lower heart chamber or in two upper heart chambers or in two lower heart chambers or in three or four heart chambers that provide synchronous pacing of upper and lower and/or right and left heart chambers as needed. Such pacing systems of the present invention overcome the problems and limitations of the multiple chamber pacing systems described above and provide a great deal of flexibility in tailoring the delivered pacing therapy to needs of the individual patient""s heart.
The isolated current replicator can be advantageously employed with conventional capacitive discharge pacing output circuits and sense amplifiers.
In addition, the use of the Isolated current replicator coupled with the pace/sense electrodes allows the morphology of spontaneous and evoked depolarizations conducted from a spontaneous or evoked depolarization in the other chamber to be analyzed to determine pathologies of the conduction pathways.
The present invention offers numerous advantages in providing right and left heart pacing to patient""s suffering from advanced congestive heart failure and exhibiting IACD, LBBB, RBBB, and/or IVCD. In the particular case where a CDW is timed out, the ability to sense a conducted evoked or spontaneous depolarization in one of the right or left heart chambers within a very short CDW from the pacing pulse or spontaneous depolarization to the other heart chamber is enhanced by use of the isolated current replicator. Longevity is enhanced by the inhibition of the delivery of pacing pulses by sensed events detected within the respective controlling CDW. The various operating modes of the IPG and each CDW and each AV delay can be programmed during chronic implantation to adjust to observed changes in the underlying electrical activation sequence as the patient""s condition improves or deteriorates.